Pin stable/wallaby jobs nodeset to Ubuntu Focal (20.04)

In 2023.1 cycle. we are moving the default distro
version of Ubuntu to Jammy (22.04)[1] so we need to pin
the nodeset for stable branch job in master gate so that
they continue run on their supporting distro version which is
Ubuntu Focal since stable/victoria.

[1] https://governance.openstack.org/tc/goals/selected/migrate-ci-jobs-to-ubuntu-jammy.html

Change-Id: Ia3f329f9f0e4d8f7541d002ad7c8e77083b44bca
1 file changed